Bi-stable reflective displays based on PDLC are known. A polymer-dispersed liquid crystal (PDLC) system contains droplets of liquid crystal material dispersed in a polymer matrix. Such systems are known in the art and have been disclosed by Doane et al. (Applied Physics Letters 48, 269, 1986), Doane et al. in U.S. Pat. No. 5,251,048, West et al. (Applied Physics Letters 63, 1471, 1993) and by Stephenson (U.S. Pat. No. 6,359,673). The PDLC may be used to create passive matrix displays on flexible substrates. See also U.S. Pat. No. 6,061,107 to Yang et al., incorporated by reference. Chiral-nematic liquid-crystals, also referred to as cholesteric liquid crystals, have the capacity of maintaining (in a stable state) one of a plurality of given states in the absence of an electric field.
West et al. (Applied Physics Letters 63, 1471 1993) disclose a PDLC based bi-stable reflective display. The device comprises droplets of chiral nematic liquid crystal (CLC) in a polymer binder coated between two transparent electrodes. The CLC material can be switched between a reflecting planar state and a weakly scattering focal conic state by application of voltage pulses of different magnitudes. The planar and focal conic states are both stable at zero applied field. However, West et al. disclose the spectrum of only a single CLC material with peak reflectivity of 564 nm and do not teach methods for obtaining an infrared-light reflecting display. They note that the domain structure of the dispersed system in the focal conic state scatters light uniformly over the visible portion of the spectrum with the back scattered intensity gradually increasing at lower wavelengths, they do not specifically teach methods to improve contrast of the display.
U.S. Pat. No. 6,061,107 to Yang et al. discloses a bi-stable polymer-dispersed cholesteric liquid crystal display having flattened domains between glass plates. The flattened domains are said to have a major axis larger than the cell thickness determined by 5-micron glass fiber spacers. The domains are dispersed in a thermoplastic polymer such as polyvinyl butyral polymer. Multicolor displays in which domains having different cholesteric liquid crystals reflecting different color light, representing different pixels, are disclosed.
Stephenson in U.S. Pat. No. 6,359,673 discloses a PDLC-based bi-stable reflective display. By combining the reflections of droplets of CLC material doped to give reflections either in the blue, green and red part of the spectrum or the blue and yellow part of the spectrum, a neutral appearance in the reflective state is obtained. Stephenson teaches that domains or droplets of liquid crystal in the light-modulating layer are smaller than the thickness of the layer so that multiple domains overlap. Stephenson further teaches that the contrast of the display may be improved by combining multiple overlapping domains comprising right-handed and left-handed chiral-nematics.
T. Kakinuma et al., in “Black and White Photo-addressable Electronic Paper using Encapsulated Cholesteric Liquid Crystal and Organic Photoconductor,” IDW '02, page 1345 to 1348 discloses a liquid crystal display using a combination of red (pink) and green capsules to create broadband. However, the imaging layer has many overlapping domains (as shown by the enlarged portion of FIG. 1).
U.S. Pat. No. 5,847,798 discloses (particularly in FIG. 7) a liquid crystal cell having multiple stable reflecting states between a colored reflecting state and a light scattering state in order to allow for a substantially white background. Under room light conditions, where light is incident on the cell from all directions, the light reflected from different domains has different colors, including wavelengths that are in the infrared spectral region, because the incident angles θ in different domains are different. As such, the light observed by a human eye is an average of the reflection bands centered at different wavelengths and has a substantially white appearance.
U.S. Pat. No. 6,034,752 to Khan et a. discloses a liquid crystal device in which the liquid crystal material has a pitch length effective to reflect radiation having both the visible and the infrared ranges of the spectrum, either in a single region (single cell) between opposing substrates or in separate regions, for example, in stacked regions in which a first cell reflects red light and a second cell reflects blue light and a third cell reflects green light. The devices described by Khan et al. have a back substrate furthest from the observer that may be painted black or a separate layer may be used to improve contrast. Thus, as shown in the figures, black paint in a background is located on the other side of the ITO electrodes from the layers with the cholesteric material. Example 1 discloses a stacked display, one cell or region reflecting infrared and one cell or region reflecting visible light. Example 2 discloses a composition for a single cell display employing a single liquid crystal material that reflects both visible and infrared radiation. The Examples do not employ domains of liquid crystal in a polymer matrix and the imaging layers are not coated on a flexible substrate.
There are two main methods for fabricating polymer-dispersed liquid crystal devices: emulsion methods and phase-separation methods. Emulsion methods have been described in U.S. Pat. Nos. 4,435,047 and 5,363,482. The liquid crystal is mixed with an aqueous solution containing a binder. The liquid crystal is insoluble in the continuous phase, and an oil-in-water emulsion is formed when the composition is passed through a suitable shearing device, such as a homogenizer. The emulsion is coated on a conductive surface, and the water evaporated. A second conductive surface may then be placed on top of the emulsion layer by lamination, vacuum deposition, or screen-printing to form a device. While the emulsion methods are straightforward to implement, droplet size distributions tend to be broad, resulting in a loss in performance. For cholesteric liquid crystal devices, also referred to herein as CLC devices, this typically means reduced contrast and brightness. Phase separation methods were introduced in an effort to overcome this difficulty.
Phase-separation methods have been outlined in U.S. Pat. No. 4,688,900 and in Drzaic, P. S. in Liquid Crystal Dispersions published by World Scientific, Singapore (1995). The liquid crystal and polymer, or precursor to the polymer, are dissolved in a common organic solvent. The composition is then coated on a conductive surface and induced to phase separate by application of ultra-violet (UV) radiation or by the application of heat or by evaporation of the solvent, resulting in droplets of liquid crystal in a solid polymer matrix. A device may then be constructed utilizing this composition.
U.S. Pat. No. 6,423,368 proposes the use of droplets of the liquid crystal material prepared using a limited coalescence process. In this process, the droplet-water interface is stabilized by particulate species, such as colloidal silica. Surface stabilization by particulate species such as colloidal silica is particularly preferred as it can give narrow size distribution and the size of the droplets can be controlled by the concentration of the particulate species employed. The materials prepared via this process are also referred to as Pickering Emulsions and are described more fully by Whitesides and Ross (J. Colloid Interface Sci. 169, 48 (1995)). The uniform droplets may be combined with a suitable binder and coated on a conductive surface to prepare a device. The process provides significant improvement in brightness and contrast over prior art processes. It also overcomes some of the problems associated with photoinitators and UV radiation, which are used in phase-separation techniques.
Commonly assigned, copending U.S. patent application Ser. No. 10/718,900 to Chari et al. shows that the maximum contrast in a bi-stable cholesteric liquid crystal mixture display prepared by the limited coalescence method is obtained when the uniform liquid crystal domains or droplets are coated as substantially a monolayer on the first conductive support. The bi-stable states in these cholesteric liquid crystal mixture displays are the planar reflecting state and the weakly scattering focal conic state. Backscattering of light from the weakly scattering focal conic state increases drastically when there is more than a monolayer of droplets between the conductive surfaces. Chari et al. disclose a liquid crystal display having an imaging layer comprising a mixture of two populations of liquid crystal domains, both populations, however, having a peak wavelength in the visible spectrum.
It would be useful to provide liquid-crystal displays that reflect in both the visible and infrared light ranges with improved contrast. It would be useful for such displays to be fabricated using simple, low-cost processes.